CO₂ and CH₄ budgets and global warming potential modifications in Sphagnum-dominated peat mesocosms invaded by Molinia caerulea

Abstract. Plant communities play a key role in regulating greenhouse gas (GHG) emissions in peatland ecosystems and therefore in their ability to act as carbon (C) sinks. However, in response to global change, a shift from Sphagnum to vascular plant-dominated peatlands may occur, with a potential alteration in their C-sink function. To investigate how the main GHG fluxes (CO₂ and CH₄) are affected by a plant community change (shift from dominance of Sphagnum mosses to vascular plants, i.e. Molinia caerulea), a mesocosm experiment was set up. Gross primary production (GPP), ecosystem respiration (ER) and CH₄ emission models were used to estimate the annual C balance and global warming potential under both vegetation covers. While the ER and CH₄ emission models estimated an output of, respectively, 376 and 7 gC m⁻² y⁻¹ in Sphagnum mesocosms, this reached 1018 and 33 gC m⁻² y⁻¹ in mesocosms with Sphagnum rubellum and Molinia caerulea. Annual modelled GPP was estimated at −414 and −1273 gC m⁻² y⁻¹ in Sphagnum and Sphagnum + Molinia plots, respectively, leading to an annual CO₂ and CH₄ budget of −30 gC m⁻² y⁻¹ in Sphagnum plots and of −223 gC m⁻² y⁻¹ in Sphagnum + Molinia ones (i.e., a C-sink). Even if, CH₄ emissions accounted for a small part of the gaseous C efflux (ca. 3 %), their global warming potential value makes both plant communities have a climate warming effect. The shift of vegetation from Sphagnum mosses to Molinia caerulea seems beneficial for C sequestration at a gaseous level. However, roots and litters of Molinia caerulea could provide substrates for C emissions that were not taken into account in the short measurement period studied here.